Transonic NLF wing aircraft configurations as described herein are desirable for efficient transonic cruise, e.g. high subsonic speeds typically above about Mach 0.80 and up to slightly above Mach 1. Principal features of the herein described configurations are low to moderate sweep, sharp or slightly blunted leading edge, and relatively thin airfoils in terms of the ratio of maximum airfoil thickness to chord (t/c). The importance of the NLF boundary layer (BL) in terms of drag reduction can be understood by considering that for typical transonic cruise flight conditions the laminar skin friction drag is approximately 10% of the turbulent skin friction drag associated with a traditional swept wing designs, for the same amount of surface area. Additionally, the transonic NLF wing configurations described herein can achieve best cruise efficiency at higher Mach numbers than possible with the swept wings typically used on high speed subsonic cruise aircraft.
For extensive NLF, the wing must have low or moderate sweep, and thus, on a purely aerodynamic basis the low sweep NLF wing should be relatively thin to limit the volume wave drag at the design cruise Mach number. On the other hand a thinner wing incurs a weight penalty, since structural weight varies inversely with wing thickness, everything else being equal, so that selection of thickness to chord ratio (t/c) is of substantial importance to optimizing the performance of such aircraft.
In previous studies, the NLF wing was designed to give best efficiency at speeds of about Mach 0.95 or higher. This work formed certain bases for U.S. Pat. No. 7,000,870, “LAMINAR FLOW WING FOR TRANSONIC CRUISE”, incorporated herein by reference. This Mach number criterion led to the provision of about 3% (0.03) as an upper limit for the span-wise average t/c ratio of the NLF wing and leading edge sweep angles of less than about 20.
However, that prior patent specified no variation of t/c with design cruise Mach number, M. Design studies have been extended to cover a range of cruise Mach numbers down to about Mach 0.80, near the maximum efficient cruise Mach number of previous subsonic aircraft with low wing sweep designed for long range. These studies showed that low or moderately swept NLF wings having average t/c up to about 0.08 (8%) would fill a gap in efficient cruise Mach number between about 0.80 and about Mach 0.95. Such wings can be designed for extensive NLF by methods described in our prior patents and the patent application referenced above (Ser. No. 12/589,424) of which this is a continuation in part.
In addition, certain design combinations of wing sweep and t/c, can enable efficient cruise Mach numbers up to about 1.05, well beyond the maximum efficient cruise Mach number of high speed, long range aircraft other than supersonic designs capable of operating at more than about Mach 1.2. Such wings were found to require average t/c ratios of about 0.03 (3%) or less, and for some missions could benefit from greater leading edge sweep than the previous limit of about 20 degrees specified in our U.S. Pat. No. 7,000,870. For example a sweep of about 30 degrees is required for an efficient cruise Mach number of 0.99 with an average t/c ratio of about 0.03 (3%). Achieving extensive NLF for such wing sweep is more difficult and some loss in LF coverage extent is inevitable.
We have found the foregoing combinations of thickness and sweep to be advantageous for efficient flight at transonic speeds and determined that these combinations have not been used or disclosed previously. For example there are many subsonic aircraft which are limited to maximum cruise speeds of less than Mach 0.80, and which utilize low sweep, but have thicker wings than the herein proposed t/c upper limit of 8%. On the other hand there are aircraft such as commercial jet airliners and high speed business jets, which are designed for efficient cruise speeds above Mach 0.8 but which have much higher than 25 degrees of wing sweep, or t/c greater than 8%. Finally there are actual and proposed supersonic aircraft such as “Concorde”, designed for cruise speeds well above Mach 1.2, which feature t/c below 3%, but use very high leading edge sweep greater than about 50 degrees.
As previously noted in prior application (Ser. No. 12/589,424) titled, LAMINAR FLOW WING OPTIMIZED FOR SUPERSONIC CRUISE AIRCRAFT, a number of considerations may drive the optimal thickness to higher values, even at the expense of a moderate increase in volume wave drag for a given design Mach number. For example the favorable pressure gradient, which stabilizes the laminar boundary layer, increases with t/c ratio, and as noted, structural weight decreases with increasing thickness. In addition, the volume wave drag attributable to the wing can be reduced by contouring the fuselage in the vicinity of the wing. Finally, the achievement of NLF on large areas of the wing surface is dependent on (a) achieving appropriate pressure gradients over the affected surfaces of the wing and (b) suitable leading edge size and shape. These pressure gradients depend not only on the local airfoil shapes, but also are influenced by the fuselage contour or contours adjacent to the wing.
There is, accordingly, a need for improvements in cruise efficiency and range of transonic aircraft, and particularly in the optimization of the airfoil shapes, thickness to chord ratios, wing sweep and aspect ratio, as well as the fuselage contours affecting both volume wave drag and NLF extent over the wing surfaces. Similar considerations can be applied to the design of horizontal and vertical tail surfaces.